Alloys of mercury, cadmium and tellurium are well known to be highly useful in fabricating infrared detectors.
Since HgTe is a semimetal (having a very small negative band gap), and CdTe has a band gap of about 1.5 eV, compositions having an extremely small and arbitrarily selectable band gap may be specified simply by varying the proportions of an alloy having the composition Hg.sub.1-x Cd.sub.x Te. Such alloys are here referred to generically as "HgCdTe". For example, the composition Hg.sub.0.8 Cd.sub.0.2 Te is a 10 micron material, that is, a composition having a bandgap approximately equal to the photon energy of infrared light having a wavelength of 10 microns. By reducing the percentage of cadmium, compositions having a smaller band gap, and therefore a longer operating wavelength, may be produced.
Applications for such a semiconductor, having a small and arbitrarily selectable bandgap, are numerous. However, since the band gap varies with the composition, it is necessary for many applications that the composition of the alloy be uniform. In addition, it is of course necessary, for use in photodetectors, to provide materials which are relatively free of physical defects. Unfortunately, the characteristics of the HgCdTe system make preparation of such alloys difficult. In particular, it is highly desirable to provide an infrared detector operating at a wavelength of 12 microns or longer. Although HgCdTe alloys are transparent down to wavelengths in the 30 micron range, it has heretofore not been practicable to reliably fabricate HgCdTe alloys for operation at wavelengths significantly longer than 10 microns.
Heretofore long wavelength detectors have also been fabricated using doped simiconductors, such as silicon. With such material, the energy states provided by the dopants within the bandgap are used to provide a small transition energy, and therefore a long-wavelength absorption. However, intrinsic long-wavelength detectors are more efficient, and have much more definite frequency characteristics, than such doped materials. The present invention aims at providing an intrinsic long-wavelength detector, which has heretofore not been practicable to provide.
As discussed in U.S. Pat. No. 3,656,944, which is hereby incorporated by reference, a uniform mixture of mercury, cadmium and tellurium is usually achieved by preparing them as a homogenous liquid mixture. However, if such a mixture is cooled slowly, the solid which freezes out will have a progressively varying composition. To avoid these differential freezing effects, one method which has been attempted in the art is to quench a homogenous liquid mixture. However, two further difficulties arise in such a quenching process. First, as with most quenching processes where the solid state is significantly denser than the liquid state, the contraction of the liquid mixture as it solidifies is likely to cause formation of voids and "pipes" (that is, longitudinal voids near the center of a cylindrical body). Second, due to the very high vapor pressure of mercury at all temperatures of interest, it is difficult to prevent mercury from escaping from the solidus-liquidus mixture into any adjacent vacant space, including voids which may be created during the freezing of the mixture. U.S. Pat. No. 3,656,944 discusses ways to minimize this escape of mercury, but the method disclosed by this patent still permits significant inhomogeneity to remain in the alloy produced, and the imprecision of this method also does not permit full exploitation of the advantages which may be obtained, as discussed above, from selecting the band gap of the material produced by controlling the exact composition of the alloy used. Other methods of making HgCdTe have also not succeeded in attaining good yield rates.
Vapor phase epitaxy of HgCdTe has also been attempted, but this approach may result in a graded composition, and is believed not to provide the advantages of the present invention. See Becla, "A Modified Approach to Isothermal Growth of Ultrahigh Quality HgCdTe for Infrared Applications", forthcoming in J. Electrochemical Soc.
A further problem with present methods of HgCdTe production is that the area of the photodetector which can be produced is limited by the maximum single-crystal size which can be provided. Since the largest single-crystal size which is currently practical in production quantities is on the order of one inch square, this places a drastic size limitation on present HgCdTe detectors.
General references on the properties of CdTe and HgTe, and of certain other analogous ternary and quaternary systems, may be found in K. Zanio, 13 Semiconductors and Semimetals (1978), especially at pages 212 and following; and Harmon, "Properties of Mercury Chalcogenides", in Physics and Chemistry of II-VI Compounds (ed. M. Aven & J. Prener, 1967); all of which are hereby incorporated by reference.
It is also frequently desirable to be able to detect the infrared spectrum of a distant object. One method for doing this is to image the same object on different detectors, each operating at different wavelengths. However, such a system requires precise optical calibration and adjustment, and, to resist decollimation, such a system must be made relatively bulky and heavy. Thus, it would be highly desirable to provide an infrared detector which could directly detect more than one wavelength on a single substrate.
It is an object of the present invention to provide HgCdTe devices suitable for use as photodetectors. It is a further object of the present invention to provide HgCdTe films suitable for use as photodetectors.
It is a further object of the present invention to provide HgCdTe devices, suitable for use as photodetectors, which have a very low density of material defects.
It is a further object of the present invention to provide HgCdTe films, suitable for use as photodetectors, which have extremely homogeneous composition.
It is a further object of the present invention to provide a method for producing HgCdTe films whereby the exact composition of the final alloy may be accurately preselected.
It is a further object of the present invention to provide HgCdTe devices, suitable for use as photodetectors, which have extremely flat surfaces.
It is a further object of the present invention to provide HgCdTe films, suitable for use as photodetectors, which have a very large area. It is a particular object of the present invention to provide HgCdTe films, suitable for use as photodetectors, which have an area significantly larger than one square inch.
It is a further object of the present invention to provide HgCdTe films, suitable for use as photodetectors, which have an extremely low density of surface defects.
It is a further object of the present invention to provide a process for manufacturing HgCdTe devices which provides an extremely high yield of satisfactory devices (i.e., number of satisfactory devices as a percentage of total devices).
It is a further object of the present invention to provide a method for manufacturing HgCdTe devices, in which the yield rate is relatively insensitive to variation in parameters in the manufacturing process.
It is a further object of the present invention to provide a process for manufacturing HgCdTe devices which does not require precise control of all manufacturing process parameters.
It is a further object of the present invention to provide a process for manufacturing HgCdTe devices which includes self-limiting steps, so that the manufacturing process achieves no further effect on the device being manufactured, once the desired end product stage has been achieved.
It is a further object of the present invention to provide HgCdTe devices for detection of very long wavelength light.
It is a further object of the present invention to provide HgCdTe devices for detection of light at wavelengths longer than 12 microns.
It is a further object of the present invention to provide HgCdTe structures having a uniform and extremely small non-zero band gap.
It is a further object of the present invention to provide a method for producing uniform films of an intrinsic semiconductor having an extremely small non-zero band gap.
It is a further object of the present invention to provide a method for producing HgCdTe films wherein first portions have a band gap corresponding to a first wavelength and second portions have a band gap corresponding to a second wavelength.
It is a further object of the present invention to provide a monolithic HgCdTe film for multi-color infrared imaging.